1. Field of the Invention
The present invention relates to an optical modulation device using a liquid crystal, e.g. a liquid crystal, display device, a liquid crystal-optical shutter array, etc. and more particularly to an optical modulation device having improved display and driving characteristics through improved initial orientation of liquid crystal molecules, and a method of producing the same.
2. Description of the Prior Art
Hitherto, liquid crystal display devices have been well known, which comprise a group of scanning electrodes and a group of signal electrodes arranged in a matrix, and a liquid crystal compound filled between both electrode groups to form a plurality of picture elements to display images or information at matrix intersecting points. For driving these display devices, there is employed a time sharing driving method comprising selectively applying address signals sequentially and periodically to the group of scanning electrodes and selectively applying certain information signals to the group of signal electrodes in a parallel fashion in synchronism with the address signals. However, these display devices and the driving method therefor have serious drawbacks which are be described below.
Namely, it is difficult to obtain a high density of picture elements or a large image area. Because of the relatively high response speed and low power dissipation, most of the liquid crystals which have been put into practice as display devices are TN (twisted nematic) type liquid crystals, as shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich "Applied Physics Letters" Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128. In liquid crystals of this type, molecules of nematic liquid crystals which show positive dielectric anisotropy under no application of an electric field form a structure twisted in the thickness direction of the liquid crystal layers (helical structure), and molecules of this liquid crystal form a structure aligned or oriented parallel to each other near the surfaces of both electrodes. On the other hand, nematic liquid crystals which show positive dielectric anisotropy under application of an electric field are oriented or aligned in the direction of the electric field, thus permitting optical modulation. When display devices are constructed in a matrix electrode arrangement using a liquid crystal of this type, a voltage higher than a threshold level required for aligning liquid crystal molecules in the direction perpendicular to the electrode surfaces is applied to an area (a selected point) where a scanning electrode and a signal electrode are selected at a time, whereas a voltage is not applied to areas (non-selected points) where scanning electrodes and signal electrodes are not selected. Accordingly, the liquid crystal molecules are stably aligned parallel to the electrode surfaces. When linear polarizers having a cross nicol relationship to each other (i.e. their polarizing axes are arranged perpendicular to each other) are arranged on the upper and lower sides of the liquid crystal cell thus formed, light is not transmitted at selected points while it is transmitted at non-selected points. Thus, the liquid crystal cell can function as an image device.
However, when a matrix electrode arrangement is formed, a certain electric field is applied to regions where a scanning electrode is selected and signal electrodes are not selected, or regions where scanning electrodes are not selected and a signal electrode is selected (which regions are called "half selected points"). If the difference between a voltage applied to the selected points and a voltage applied to half selected points is sufficiently large, and a voltage threshold level required for allowing liquid crystal molecules to be aligned or oriented perpendicular to an electric field is set to a value therebetween, display devices normally operate. However, in fact, as the number (N) of scanning lines increases, a time (duty ratio), during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned, decreases with a ratio of 1/N. Accordingly, the larger the number of scanning lines, the smaller is the voltage difference of an effective value applied to a selected point and non-selected points when scanning is repeatedly effected. As a result, this leads to unavoidable disadvantages of lowering of image contrast or occurrence of interference or crosstalk. These phenomena are regarded as essentially unavoidable problems appearing when a liquid crystal having no bistability (i.e. liquid crystal molecules are horizontally oriented with respect to the electrode surface in their stable state and are vertically oriented with respect to the electrode surface only when an electric field is effectively applied) is driven (i.e. repeatedly scanned) by use of a time storage effect. To overcome these drawbacks, the voltage averaging method, the two-frequency driving method, the multiple matrix method, etc. have been already proposed. However, these methods do not sufficiently overcome the above-mentioned drawbacks. As a result, development of a large image area or a high packaging density of the display elements has been delayed because it is difficult to sufficiently increase the number of scanning lines.
Meanwhile, turning to the printer field, as means for obtaining a hard copy in response to input electric signals, a Laser Beam Printer (LBP) providing electric image signals to an electrophotographic photosensitive member in the form of light is excellent in terms of the density of picture elements and the printing speed.
However, the LBP has drawbacks as follows.
(1) The device becomes large in size.
(2) There are high speed mechanically movable parts, such as a polygon scanner, resulting in noise and requiring strict mechanical precision, etc.
In order to eliminate the drawbacks stated above, a liquid crystal shutter-array for changing electric signals to optical signals is proposed. When picture element signals are given with a liquid crystal shutter-array, for instance, 4000 signal generators are required for writing picture element signals a length of 200 mm at a ratio of 20 dots/mm. Accordingly, in order to independently feed signals to respective signal generators, wiring of lead lines for feeding electric signals to all of the respective signal generators is required, resulting in difficulties in production.
In view of this, another attempt was made to apply image signals corresponding to one line in a time-sharing manner with signal generators correspondingly divided into a plurality of lines. With this attempt, signal feeding electrodes can be common with the plurality of signal generators, thereby remarkably reducing the amount of wiring required. However, if an attempt is made to increase the number (N) of lines using a liquid crystal showing no bistability as is usually practiced, a signal "ON" time is substantially reduced to 1/N. This results in difficulties that the amount of light obtained on a photo-sensitive member is lowered, crosstalk occurs, etc.
To overcome the drawbacks of such prior art liquid crystal devices, the use of liquid crystal devices having bistability has been proposed by Clark and Lagerwall (e.g. Japanese Laid-Open Patent Appln. No. 56-107216, U.S. Pat. No. 4,367,924, etc.). In this instance, as the liquid crystals having bistability, ferroelectric liquid crystals having chiral smectic C-phase (SmC*) or H-phase (SmH*) are generally used. These liquid crystals have bistable states of first and second stable states with respect to an electric field applied thereto. Accordingly, in contrast to optical modulation devices in which the above-mentioned TN-type liquid crystals are used, the bistable liquid crystal molecules are oriented to first and second optically stable states with respect to one and the other electric field vectors, respectively. The characteristics of the liquid crystals of this type are such that they are oriented to either of two stable states at an extremely high speed and the states are maintained when an electric field is not supplied thereto. By making use of such characteristics, these liquid crystals having chiral smectic phase can essentially diminish a large number of problems of the prior art TN-type devices. This is described in detail hereinafter in relation to the present invention.
However, in order that an optical modulation device in which a liquid crystal having bistability is used can realize desired driving characteristics, it is required that the liquid crystal disposed between a pair of parallel base plates has a molecule arrangement such that the molecules can effectively be switched between the two stable states independent of the application of an electric field. For instance, in connection with ferroelectric liquid crystals having SmC*- or SmH*-phase, it is required that there is formed a region (monodomain) where liquid crystal layers having SmC*- or SmH-phase are vertical to the surface of base plates, i.e. the liquid crystal axis is aligned substantially in parallel therewith. However, with optical modulation devices in which a liquid crystal having bistability is used, the orientation of the liquid crystal having such a monodomain structure has not satisfactorily been formed, thus failing to obtain sufficient display characteristics.
For instance, in order to give such an orientation, Clark et al., have proposed a method of applying an electric field, a method of applying shearing stress, and a method of arranging ridges in parallel with each other at a small interval between base plates, etc. However, these methods could not necessarily provide satisfactory results. For instance, the method of applying an electric field requires a large scale apparatus and is not compatable with a thin layer cell having excellent operational characteristics. Further, the method of applying a shearing stress is not compatible with a method of filling a liquid crystal after a cell is prepared. Furthermore, the method of arranging parallel ridges within the cell cannot provide a stable orientation effect by itself.
Meanwhile, in the liquid crystal device in which the above mentioned TN-type liquid crystal is used, in order to form a monodomain of liquid crystal molecules in parallel with the surface of a base plate, for instance, a method of rubbing the surface of the base plate with a cloth, etc., or a method of effecting oblique vapor deposition of SiO, etc. has been used. In accordance with the rubbing method, liquid crystal molecules assume a lowest energy (i.e., stable) state where they align preferentially along the rubbing direction, because the molecules are orientated due to grooves formed by rubbing on the surface of a base plate, or a combined effect of the grooves and a certain effect produced by rubbing, the nature of which has not necessarily been clarified. Thus, a certain "wall effect" for preferentially orienting liquid crystals in one direction is given to such a rubbed surface. A structure having a plane to which such wall effect is given is shown in, e.g. Canadian Pat. No. 1010136, etc. by W. Helfrich and M. Schadt. In addition to the rubbing method for forming the wall effect, another method is employed, in which a structure having a plane formed by oblique vapor deposition of SiO or SiO.sub.2 on a base plate is used, and the plane having a uniaxial anisotropy of SiO or SiO.sub.2 has a wall effect for preferentially orienting liquid crystal molecules in one direction.
As stated above, the alignment or orientation control method, e.g. the rubbing method or the oblique-deposition method, is one of the preferable methods for producing liquid crystal devices. However, if the alignment control is implemented to liquid crystals having bistability by these methods, a plane having a wall effect for preferentially aligning a liquid crystal only in one direction is formed, thereby deteriorating the desirable characteristics of the bistable liquid crystals such as bistability with respect to an electric field applied thereto, high responsiveness or monodomain forming ability. Accordingly, in the field of ferroelectric liquid crystals attention has not been drawn to the employment of the rubbing method or the oblique-deposition method as a means for controlling alignment of liquid crystals.